Thin film deposition techniques are widely used in the manufacturing of microelectronic devices to form a coating on a workpiece that closely conforms to the surface topography. The size of the individual components in the devices is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) is increasing. The size of workpieces is also increasing to provide more real estate for forming more dies (i.e., chips) on a single workpiece. Many fabricators, for example, are transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms and corners in deep depressions that have very small openings.
One widely used thin film deposition technique is Chemical Vapor Deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a thin solid film at the workpiece surface. The most common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction.
Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then a high workpiece temperature is needed to achieve a reasonable deposition rate. Such high temperatures are not typically desirable because heating the workpiece can be detrimental to the structures and other materials that are already formed on the workpiece. Implanted or doped materials, for example, migrate in the silicon substrate when a workpiece is heated. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the substrate. This is not desirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used. Thus, CVD techniques may not be appropriate for many thin film applications.
Atomic Layer Deposition (ALD) is another thin film deposition technique that addresses several of the drawbacks associated with CVD techniques. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules Ax coats the surface of a workpiece W. The layer of Ax molecules is formed by exposing the workpiece W to a precursor gas containing Ax molecules, and then purging the chamber with a purge gas to remove excess Ax molecules. This process can form a monolayer of Ax molecules on the surface of the workpiece W because the Ax molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of Ax molecules is then exposed to another precursor gas containing By molecules. The Ax molecules react with the By molecules to form an extremely thin solid layer of material on the workpiece W. The chamber is then purged again with a purge gas to remove excess By molecules.
FIG. 2 illustrates the stages of one cycle for forming a thin solid layer using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor Ax, (b) purging excess Ax molecules, (c) exposing the workpiece to the second precursor By, and then (d) purging excess By molecules. In actual processing several cycles are repeated to build a thin film on a workpiece having the desired thickness. For example, each cycle may form a layer having a thickness of approximately 0.5-1.0 Å, and thus it takes approximately 60-120 cycles to form a solid layer having a thickness of approximately 60 Å.
FIG. 3 schematically illustrates an ALD reactor 10 having a chamber 20 coupled to a gas supply 30 and a vacuum 40. The reactor 10 also includes a heater 50 that supports the workpiece W and a gas dispenser 60 in the chamber 20. The gas dispenser 60 includes a plenum 62 operatively coupled to the gas supply 30 and a distributor plate 70 having a plurality of holes 72. In operation, the heater 50 heats the workpiece W to a desired temperature, and the gas supply 30 selectively injects the first precursor Ax, the purge gas, and the second precursor By as shown above in FIG. 2. The vacuum 40 maintains a negative pressure in the chamber to draw the gases from the gas dispenser 60 across the workpiece W and then through an outlet of the chamber 20.
One drawback of ALD processing is that it is difficult to avoid mixing between the first and second precursors in the chamber apart from the surface of the workpiece. For example, a precursor may remain on surfaces of the gas dispenser or on other surfaces of the chamber even after a purge cycle. This results in the unwanted deposition of the solid material on components of the reaction chamber. The first and second precursors may also mix together in a supply line or other area of a reaction chamber to prematurely form solid particles before reaching the surface of the workpiece. Thus, the components of the ALD reactor and the timing of the Ax/purge/By/purge pulses of a cycle should not entrap or otherwise cause mixing of the precursors in a manner that produces unwanted deposits or premature reactions.
Another drawback of ALD processing is that the film thickness may be different at the center of the workpiece than at the periphery. To overcome this problem, the center of some distributor plates do not have any holes 72. In practice, however, this may cause the film at the center of the workpiece to be thinner than the film at the periphery. Moreover, the center portion of such plates may become coated with the solid material because it is difficult to purge all of the precursors from this portion of the gas dispenser 60 during normal purge cycles. Therefore, there is a need to resolve the problem of having a different film thickness at the center of the workpiece than at the periphery.